Voltage regulator circuits are currently and widely used to supply a set voltage level to electronic devices. When used with semiconductor circuits and modules, such as DRAMs (Dynamic Random Access Memories) that require a wide bandwidth, high current, and loop stability while supplying current to high capacitive loads, known voltage regulator circuits have been found to be inadequate especially for applications that use input voltages below 5 volts or output voltages below 3 volts.
A common way of maintaining loop stability in low voltage applications is by using either excessively large chip areas or by accepting lower circuit performance. An attempt to provide better circuit performance used a lag-lead resistor-capacitor arrangement commonly known as a Miller compensation circuit in which a resistor and a capacitor are arranged in series between the gate and the output of the output transistor.
These Miller compensation circuits have problems especially when used with large integrated circuits, logic or memory, having large load capacitances, such as DRAMs or BASIC, and requiring high current and fast response times. In DRAMs, the Miller compensation circuit required becomes unacceptably large resulting in an unacceptable bandwidth and the introduction of additional low frequency poles into the system which further adversely affect the performance of the voltage regulator.
Also, in such integrated circuit applications, low input voltages with a low input to output voltage requires a large voltage swing across the gate to source of the output transistor further limiting the size and type of capacitive device that can be used. For example, the large voltage swing requires the Miller capacitor to function with reversible polarity thus precluding the use of devices such as thin oxide capacitors typically used in integrated circuits and requiring instead that the capacitor to be a metal capacitor which is substantially larger than such thin oxide capacitors.
Additionally, in such low voltage applications, cascade stages, which would permit the use of still smaller compensation capacitors, can not be used to boost the current source impedance. Since such regulators require a large output transistor and the Miller compensation scheme requires the Miller capacitance to be substantially larger than the gate-drain capacitances of the output transistor, the Miller compensation scheme cannot be implemented in a practical design.
Still another prior art attempt employed respective parallel resistor-capacitor arrangements coupled to the source of each of the differential input transistors to provide lead compensation. In both this latter arrangement and in the Miller compensation scheme, the large output load capacitance is used as a dominant pole and the output drive circuit contributes a second pole to the circuit. Although this regulator works well with a 5 volt input voltage and a 3.3 output voltage it fails at lower voltages for it cannot maintain sufficient voltage across the capacitors and still provide an adequate voltage swing on the gate of the output device.
Accordingly when faced with these problems, the prior art could only degrade the performance of the entire integrated circuit and thus limit the conditions that would generate these problems.
Therefore to achieve the smallest chip size and to utilize the full performance capabilities of such integrated circuits, there now exists a need for a new and improved voltage regulator circuit which avoids all the above described problems associated with the prior art low voltage regulators used in large integrated circuits while achieving the full performance of the circuit, especially when employing input voltages below 5 volts.
The present invention avoids all the above described problems associated with the prior art circuits and achieves small size and full circuit performance at low input voltages, especially input voltages below 5 volts, and other desirable results by comparing, in a differential amplifier, the regulator circuit output to a reference voltage and generating a differential current to provide a current drive to the gate of a control transistor, which is, in turn, driven by second and third current sources capable of driving the control transistor from ground to voltage.